The present invention relates to a horizontal-axis aerator and a purification system employing such aerator, particularly for use in the purification of waste water.
The biological purification of waste water loaded with organic pollution necessitates the introduction of oxygen into the water to assure the respiration and purifying action of the bacterial mass contained in the waste water. Introduction into the waste water of oxygen from the air above the upper surface of the waste water to be treated, and the dissolving of such oxygen into the waste water can be accelerated by means of a device located near the upper surface level of the waste water to be treated which by mechanical action causes an increase in the surfaces of contact between the air and the waste water.
A number of devices based on this general principle are known, and are generally termed mechanical aerators. Some such known aerators have vertical axes, are called aeration turbines and are generally employed in treatment tanks having a depth from 2.50 to 6.0 m. Other known devices have horizontal axes and are generally employed in shallower tanks, i.e. of a depth of from 1.3 to 4.0 m, and constructed as a channel having a closed loop configuration. Such horizontal axis aerators generally consist of a cylinder having a horizontal axis and equipped with elements forming a multiplicity of blades.
Such known devices, when utilized for the purification of residual water, must achieve both an oxygenation function and a mixing and homogenization function of the waste water contents of the particular treatment tank. Indeed, in order to achieve effective contact between the microorganisms and the pollutants, it is essential that all of the particles contained in the waste water to be treated be held in suspension, simultaneously with oxygenation.
When the treatment tank is constructed in the form of a channel having a closed loop configuration, mixing is obtained by assuring an adequate horizontal circulation speed for the liquid within the tank. For reasons of investment savings, generally both the oxygenation and mixing functions are assured by means of a single horizontal axis aerator arranged across one or all of the branches of the channel having the closed loop configuration.
However, while the needs for suspension and mixing remain substantially constant, in contrast the amounts of oxygen to be introduced per unit of waste water volume in the biological treatment tank may vary within very broad limits. Oxygenation needs in fact vary from one installation to another, and within a single installation from one hour of the day to another. That is, in so called "heavy load" purification stations, for which the quantity of pollution may be expressed as daily weight of BOD.sub.5 added per unit of tank volume, the oxygenation needs are sharply greater per unit of tank volume than in lighter load installations, such as for example the so called "extended aeration" stations which may also provide nitrification of the ammoniacal and organic nitrogen contained in a raw effluent.
Furthermore, in a given installation and in a given day the hourly pollution flow, which may be expressed for example as hourly weight of BOD.sub.5 supplied, arriving at the intake of the biological treatment tank of the purification installation of a community may vary within very broad limits. The pollution flow may frequently be ten to twenty times higher during some peak hours of the day than during slack night hours. Oxygenation needs therefore vary within very broad limits throughout the day.
It is thus worthwhile to have aerators having a mixing or "pumping" capacity which is high even for operating speeds bringing in only a low oxygen supply. This avoids keeping aerators in service for purely hydraulic reasons when the oxygen needs of the particular bacterial mass do not necessitate it. To date, efforts have primarily been directed to improving the specific oxygen supply, which is represented by the relationship of the weight of oxygen introduced divided by the energy consumed, of horizontal-axis aerators by dividing the amount of water displaced by the action of the device and by facilitating the introduction of air into the liquid as soon as the aerator has been placed in contact with the liquid, i.e. upstream from the aerator. The design and construction of the blades on the surfaces of most horizontal-axis aerators is carried out with a two fold goal. That is, the number of blades is quite large, i.e. several tens of blades per meter of the device, and the blade shape includes many sharp angles. The blades are often wider at the periphery of the aerator than at the area of attachment of the blades to the cylinder. The unit surface of each blade is low, on the order of one square decimeter. The blades are most often placed to extend along diametral planes passing through the axis of the cylinder.
However, the known designs of horizontal-axis aerators have certain inherent disadvantages. Thus, the pumping capacity of such known aerators is low. To ensure a minimum mixing, known aerators often consume a level of power greater than required by the oxygenation needs. This leads to the risk of over-oxygenating the activated sludge liquid. The small dimensions of the blades of known horizontal-axis aerators results in a major portion of the volume of water displaced by each blade escaping over the lateral edges of such blade and being picked up by following blades. Accordingly, the real or true efficiency of the pumping operation, intended to assure both circulation within the channel and renewal of those liquid portions or streams which are poorest in oxygen, is relatively low.
Also, downstream of the aerator, that liquid displaced by the blades is lifted upwardly and is uselessly projected relatively highly above the surface of the liquid. This is an unnecessary expenditure of energy. Furthermore, it often occurs that a substantial amount of the flow is driven by the blades in a manner such that it is returned or recycled upstream of the aerator. This phenomenon is a result of a combination of the driving effect of the blades and centrifugal forces. Such recycled flow results in a reduction in the real or actual pumping capacity of the aerator. Furthermore, such recycled flow has a high dissolved oxygen content which is unfavorable to a major supply of oxygen from the atmosphere.
Yet further, the attachment to the cylinder of a multiplicity of blades of small dimensions with high height to width ratios, for example on the order of from 2 to 5, is a delicate operation, and the resistance of such relatively small blades to repeated bending stresses during operation of the aerator is limited.